Most drilling machines are equipped with pneumatic motors providing the rotation of the drill spindle, as well as the various forward movements of the spindle. A machine of this type is shown by FR 2,680,329, which provides a pneumatic motor for rotating the spindle, and an electric motor for forward movements requiring less power.
The machine is positioned precisely, on a workpiece, using a drilling screen, forming a jig, the screen being positioned and fastened precisely on the piece. The machine includes a jig bushing for guiding the machining tool, and means for gripping and locking the jig bushing in an opening of the screen, to fasten the machine on the screen.
The fastening of the machine on an “airplane” positioning screen is advantageously obtained using a fastening device commonly referred to as “Concentric Collet” comprising an expansible clip engaged in the opening of the screen to provide clamping of the jig bushing and locking in the opening by a pneumatic-mechanical system of the jack and lever arm type.
The clamping force of the fastening device, in particular of the Concentric Collet type, is an important parameter in the quality of the drilling operation to be performed.
The clamping forces of the fastening device cause a compensating force along the drilling axis. The value of this force must be limited as closely as possible in order to prevent permanent micro-deformations of the outer skin of the aircraft.
It is desirable to be able to adapt the clamping force without altering the mechanical configuration of the machine, which is difficult with the pneumatic jack.
It is further desirable to eliminate, or at least limit, problems during the loosening phase. In particular, in the case of a corner system like those used in a fastening device of the “Concentric Collet” type, jamming may occur.